Time-domain systems are important analytical tools for measuring properties of an object. Recently, terahertz systems known as terahertz time-domain spectrometers (THz-TDS) have been developed. These systems often use visible to near-infrared laser pulses each lasting only 10 to several hundred femtoseconds to electromagnetic pulses (“T-rays”) that each last for about a picosecond. T-rays can be transmitted through various objects, using an imaging system of lenses and mirrors to focus or collimate the T-rays. As the T-rays pass through the object under test, they are typically distorted. These changes in the T-ray signals can be analyzed to determine properties of the object. Materials can be characterized by measuring the amounts of distortion—from absorption, dispersion and reflection—of the T-rays passing through to a detector. A digital signal processing system takes the digitized data from the THz detector and analyzes the data in either the spectral or temporal domain.
Because many compounds change T-rays in characteristic ways (e.g., absorption or dispersion), molecules and chemical compounds show strong absorption lines that can serve as “fingerprints” of the molecules. T-ray spectroscopy can distinguish between different chemical compositions inside a material even when the object looks uniform in visible light. A terahertz sensor for instance can be employed to measure caliper, moisture, and basis weight of paper whose thickness is similar to the wavelengths of T-Rays.
The precision of amplitude and phase measurements in time-domain (terahertz) spectroscopy (THz-TDS) is often limited by noise in the system. It has been demonstrated that the dominant types of noise present in THz-TDS are often time base and amplitude jitter characterized by pulses traveling through the same material (or air) which reach the detector at slightly different times and with slightly different amplitudes due to fluctuations in environmental parameters (e.g., temperature fluctuations or vibrations) or hardware errors (e.g., encoder errors in the delay line). In some specific THz-TDS systems, jitter makes a significant contribution to the noise and therefore limits the measurement precision of the system. In other THz-TDS systems, it is the multiplicative noise (i.e., amplitude noise), which comes primarily from the laser that is the main source of imprecision.
U.S. Pat. No. 8,378,304 to Mousavi et al. discloses an apparatus for implementation of time-domain spectroscopy that creates a continuous set of reference pulses whereby a sample pulses' phase and amplitude can be tracked and corrected. The apparatus can be readily adopted into existing time-domain spectrometers where both amplitude and phase are of interest. A feature of the apparatus is that when it is employed in a THz-TDS, the effect of jitter can be significantly reduced.
U.S. Pat. No. 8,638,443 to Haran and Savard discloses a method, for compensating for errors in spectrometers, that includes measuring at least a portion of a path length for a signal traveling through the spectrometer during a measuring scan of a material. The detector signal corresponding to the measurement scanner is generated. Compensation for errors in the detector and signal is provided based on the measurement path length.
Typically, on-line spectrometer sensor devices are operated to periodically traverse, or “scan,” traveling webs of sheet material during manufacture. Scanning usually is done in the cross direction, i.e., in the direction perpendicular to the direction of sheet travel. These sensors typically employ single or double sided packages which traverse the width of the sheet, guided on rail systems affixed to stiff beam structures. The accuracy of the sensor system is related to the relative x, y, and z displacement alignment between upper and lower sensor halves. The scanner heads can become misaligned up to a few millimeters or more between forward and backward scanning directions. Even small displacements can adversely affect the detected THz signal.